312 s. H ATTA : 



elapsed, and, so far as I am aware, no important fact has been 

 added during the interval by any renewed researches. I need 

 not, therefore, apologise for the ^publication of the present paper 

 which embodies the results of my study on the subject during 

 the last few years. 



The investigatio]! of a longitudinally stretched, or a meta- 

 merically arranged, organ-system such as the neural canal, the 

 chorda, the pronephros, &c., is rendered peculiarly difficult 

 in Petromyzon by the fact that the longitudinal axis of the 

 embryo in early stages describes a semi-circle. Some sections 

 in a series of cross-sections of such an embryo are therefore 

 unavoidably cut in planes which meet the longitudinal axis 

 of the embryo in variable degrees of inclination ; consequently a 

 structure stretching in the direction of this axis is cut through 

 obliquely, as, for instance, the neural cord shown in figs. 2 and 

 3, PL xvii. The vertical dimension of the cord is not in reality as 

 long as is represented in these figures. To gather accurate notions 

 of the form, the position, &c., of a given structure, therefore, 

 it is necessary to compare series of sections of two or more 

 embryos of as nearly the same age as possible. Further, the 

 difficulty of observation is greatly increased by the crowd of 

 yolk -granules in cells, especially by their reaction against stain- 

 ing fluids. Certain fluids such as haematoxylin, borax- carmine, 

 &c., either stain diffusely all the parts, or act on the granules 

 more intensely than on the other contents of cells, so that we 

 can not discriminate different kinds of tissues. This difficulty 

 was, however, obviated b}' employing picro-carmine. The 

 embryos were stained in toto in this fluid, decolorized to the 

 proper degree in acid-alcohol, and then washed in 909^ alcohol. 

 In the sections of specimens thus prepared, the histological struc- 



